This invention relates to a hydraulic linear actuation system, and in particular to a two-stage actuation system for use as a governor in a hydropower turbine.
Turbines which are driven by water under hydrostatic pressure to drive power producing generators must be operated at a specified constant rotational speed in order to generate constant frequency AC power. However, when the load fed by the generator increases the turbine will slow down, thereby decreasing the system frequency, and as the load decreases the turbine will speed up, thereby increasing the system frequency, unless the amount of water being fed to the turbine is changed in response to the change in the load. To this end power generation turbines have flow control gates at their inlets which can be opened to increase water flow to the turbine, and closed to decrease water flow to the turbine. The position of these gates has traditionally been controlled by a governor system utilizing a double-acting hydraulic piston and cylinder assembly operated by a servo valve that regulates the flow of hydraulic fluid to the piston and cylinder assembly. The servo valve in turn is controlled by an electronic controller that monitors the turbine rotational speed.
As hydropower turbines have become larger and larger, these governor systems have become increasingly expensive. When a generator first comes on line it must be brought from an essentially no-load condition to operating load in a manner of seconds or it will be destroyed. Thus, the flow control gates must be opened quickly during this transition period which requires a large piston and cylinder assembly that is supplied hydraulic fluid at a high rate. Since the servo valve which operates the piston cylinder also must accommodate this high flow rate, it must be quite large and large servo valves are extremely expensive.
The subject invention overcomes the foregoing shortcomings of the prior art hydropower turbine governor systems by utilizing a servo valve which can flow only a portion of the requirements of the piston and cylinder assembly, for controlling the piston and cylinder assembly when the turbine has reached its normal operating condition. A low flow servo valve is sufficient for this purpose since under normal operating conditions load changes are relatively small, and low flow rates of hydraulic fluid to the piston and cylinder assembly is all that is required to maintain generator synchronization.
The subject invention also provides high flow pressure and return conduits which extend from the source of pressurized hydraulic fluid and the hydraulic reservoir to the ends of the piston and cylinder assembly, bypassing the servo valve. Rather than placing a large volume servo valve in the high flow conduits, which, when combined with the low flow rate servo valve, would be nearly as expensive as the single large servo valve in the prior art systems, a lower cost proportional throttling valve is placed in the high flow return line. The throttling valve is actuated by the same controller that actuates the servo valve. The throttling valve is operated in combination with the low flow servo valve to control the piston and cylinder under high flow conditions. Thus, the system achieves the fine control of the lower capacity servo valve along with the high flow capability of the larger capacity throttling valve at considerably less expense than would result from using a large capacity servo valve.
In order to switch the source of pressurized hydraulic fluid between the rod end and blind end of the piston and cylinder assembly, and thus achieve two-way travel of the rod, a hydraulic logic switching circuit is utilized. The hydraulic logic circuit includes a first high flow pressure conduit which extends between the source of pressurized hydraulic fluid and the rod end of the piston and cylinder assembly and has a first logic valve located in it. A second high flow pressure conduit extends between the source of pressurized hydraulic fluid and the blind end inlet of the piston and cylinder assembly and has a second logic valve located in it. In addition, a first high flow return conduit, which extends between the hydraulic system reservoir and the rod end inlet of the piston cylinder, has a third logic valve located in it, and a second high flow return conduit, which extends between the hydraulic system reservoir and the blind end inlet of the piston and cylinder assembly, has a fourth logic valve located in it. The four logic valves are pilot operated, and a double-acting solenoid, which is activated by the controller whenever the proportional throttling valve is in operation, either opens the first and fourth logic valves and closes the second and third logic valves, or opens the second and third logic valves and closes the first and fourth. In the former situation pressurized hydraulic fluid flows to the rod end of the piston and cylinder assembly, and hydraulic fluid is discharged from the blind end through the proportional throttling valve, and the rod is retracted. In the latter situation the reverse occurs and the rod is extended. In either case the combined capacity of the servo valve and the proportional throttling valve is available to supply fluid to the piston and cylinder assembly.
The governor of the subject invention also includes a safety shutdown that causes the piston and cylinder assembly to immediately retract, and thus close the flow control gates and shut down the turbine, whenever power to the controller is interrupted. The safety shutdown includes a first bypass conduit that extends between the source of pressurized hydraulic fluid and the rod end inlet of the piston and cylinder assembly and has a first normally open shutdown valve located in it. A second bypass conduit extends between the reservoir and the blind end inlet of the piston and cylinder assembly and has a second normally open shutdown valve located in it. The first and second shutdown valves are pilot operated and a solenoid valve, which is actuated by the controller, provides pressure to their pilot ports when the controller is receiving power, and vents their pilot ports to the reservoir when the controller loses power. Thus, upon power loss to the controller the shutdown valves are opened and pressurized hydraulic fluid from an accumulator that is associated with the hydraulic system is directed to the rod end of the piston and cylinder assembly and the blind end is vented to retract the rod and close the flow control gates.
In addition, the safety shutdown includes vented pilot-to-close check valves in the hydraulic fluid lines associated with the servo valve. These valves are provided pilot pressure from the same solenoid valve that provides pilot pressure to the first and second shutdown valves, and thus are open when the controller has power and close when the controller loses power, to isolate the servo valve from the piston and cylinder assembly upon power loss.
Accordingly, it is a primary objective of the present invention to provide a two-stage hydraulic positioning system for a hydropower turbine governor in which a servo valve capable of flowing only a portion of the flow capacity of the governor piston and cylinder assembly is used to control the piston and cylinder assembly during normal operation of the turbine.
It is a further object of the present invention to provide such a device in which a separate larger flow capacity proportional throttling valve is used in conjunction with the lower flow capacity servo valve to control the piston and cylinder assembly during high flow conditions.
It is a further object of the present invention to utilize a hydraulic logic circuit in conjunction with the proportional throttling valve to control the direction of the piston and cylinder assembly.
It is a still further object of the present invention to provide a safety shutdown which causes the piston and cylinder assembly to be retracted when there is an interruption to the power which operates the system.
It is a yet further object of the present invention to provide such a safety shutdown which isolates the servo valve from the piston and cylinder assembly when there is an interruption of power to the system.